The major market regions for liquid CO2 are the United States, Western Europe, and Japan with the United States being the largest consumer. The major CO2 end-use is for food processing and carbonated beverage production.
CO2 is usually recovered as a byproduct from bioethanol production and catalytic steam reformation of natural gas followed by the water-shift reaction to produce anhydrous ammonia. In the United States, there is increasing CO2 availability from bioethanol production due to the need for “green” transport fuels.
New bioethanol CO2 sources are increasingly located west of the Mississippi River due to corn feedstock availability, whereas significant liquid CO2 demand is in the densely populated Northeastern, Southeastern, and Southern states.
Most of the Kraft paper pulp mills in the United States are located in proximity to regions of high liquid CO2 demand. It would be beneficial if new CO2 producing sources could be created within these high need regions.
Further, many global paper mills have Precipitated Calcium Carbonate (PCC) “satellite plants” that supply this important paper filler to papermakers. PCC production requires carbonating industrial slaked lime using CO2 contained in adjacent pulp mill lime mud calciner off-gases or paper mill boiler stack gases. There are supply reliability, quality, and cost issues associated with this approach such that a more reliable, higher quality and consistent CO2 feedstock source would be attractive.
Also, there is a growing need for CO2 within Kraft paper pulp mills to precipitate organic lignins from aqueous “black liquor” fuel streams normally supplied to the mill's chemical recovery boiler. This de-bottlenecks the boiler while creating a valuable, new carbon-neutral biomass derived fuel that can displace fossil fuels.
The Kraft pulp and paper industry is also a major energy consumer, with the majority of that need being met by low cost, carbon-neutral, biomass and biomass related fuels. The conventional lime mud calcination process has, however, not easily been converted to biomass fuels and remains a conspicuous consumer of high cost, global warming fossil fuels. In the United States, there are many Kraft paper pulp mills rated at approximately 1000 air dried tons per day (adtpd) bleached pulp production with each requiring about 320 tpd of calcined lime mud production at an annual natural gas and oil consumption of approximately 625 billion Btus. At 2014 energy prices this is approximately US $2.5 million per year per 1000 adtpd Kraft pulp mill.
It would also be useful to regenerate concentrated CO2 from more dilute CO2 sources as the need for large scale “greenhouse-gas” capture and sequestration projects develops. One capture process (www.carbonengineering.com) utilizes sodium or potassium hydroxides to capture dilute CO2 from ambient air and could benefit from the regeneration of concentrated CO2 from recausticizing process calcium carbonate.
The Bayer process that produces “pot-line grade” alumina feedstock from bauxite ore for aluminum metal production consumes significant amounts of caustic soda and rejects large amounts of caustic contaminated gangue known as “red mud”. Technology developed by Alcoa utilizes CO2 to neutralize this caustic contaminant and reduce land fill pollution could benefit from the present invention.
In the Kraft paper pulping process, cellulosic wood chips are mixed with aqueous cooking liquor (a.k.a. “white liquor”) composed primarily of sodium hydroxide (NaOH), sodium sulfide (Na2S), sodium carbonate (Na2CO3) and sodium sulfite (Na2SO3). This mixing occurs in a “digester” vessel at a temperature and pressure satisfactory to separate the cellulosic fiber from the natural lignins that bind such fibers.
The liberated fiber is separated from the resultant “black liquor” and is subsequently washed, bleached (or remains unbleached) and is eventually transformed into numerous paper grades.
The separated black liquor contains, aside from the original white liquor chemicals, lignins and other organic matter that previously bound the cellulosic fiber. In order to recover and recycle these costly pulping chemicals, as well as produce valuable steam and power from the contained organic lignins, the black liquor is concentrated in multiple-effect evaporators and delivered as a concentrated fuel to a chemical recovery boiler.
This chemical recovery boiler combusts the organics under unique oxidizing/reducing conditions to produce both superheated high-pressure steam and a molten inorganic ash (“smelt”) consisting primarily of Na2S and Na2CO3. The co-produced high-pressure steam is subsequently exhausted via a steam turbine/generator to produce mill power and low-pressure process steams.
The smelt is drained from the chemical recovery boiler and quenched in water to create “green liquor.” This green liquor is subsequently clarified and filtered to remove insoluble impurities whereupon it is delivered to the “slakers” to initiate conversion of the dissolved Na2CO3 into NaOH required for the white liquor. This slaking process utilizes calcium oxide CaO (a.k.a. calcined lime mud, or re-burned lime, or calcine) to convert Na2CO3 into NaOH via the following two consecutive reactions:CaO(s)+H2O→Ca(OH)2(s)  1)Na2CO3(aq)+Ca(OH)2(s)→2NaOH(aq)+CaCO3(s)  2)
The slaker product slurry, consisting of all the chemicals involved in reactions 1 and 2, is fed to subsequent recausticizers where reaction 2 nearly proceeds to completion with some residual Na2CO3 remaining in the white liquor. The resultant white liquor mix of NaOH, Na2S, Na2CO3, and Na2SO3 is physically separated from the precipitated calcium carbonate (CaCO3), or “lime mud” and recycled to the digester to initiate the pulping process.
The lime mud is further water washed and filtered to recover as much white liquor as economically possible before being fed to a rotary kiln calciner which converts the lime mud into calcined lime mud, (CaO and impurities) for recycle to the slakers. During the washing/filtering process, trace amounts of residual Na2S are air oxidized into more stable sodium thiosulfate (Na2S2O3) to reduce noxious total reduced sulfur (TRS) compounds which can be created in and emitted by the rotary kiln.
The highly endothermic lime mud calcination reaction typically occurs in a rotary kiln, although fluidized bed calciners have also been utilized. Use of an external lime mud flash drying (LMD) process, when combined with the rotary kiln, creates the current “state-of-the-art” optimized energy consuming lime mud calcination process.
The first fluidized bed (“FluoSolids”) lime mud calcination process was commercially introduced in 1963. It initially gave significant competition to rotary kilns due to its relatively lower fuel consumption, higher product quality, and compactness. It fell into disuse, however, as rotary kiln/LMD technology re-captured the fuel economy lead and FluoSolids installations experienced operability issues and an inability to economically operate at the high unit capacities required by a “world-class” Kraft paper pulp mill.
The kiln's primary endothermic (TR=25° C.) calcination reaction is:CaCO3(s)→CaO(s)+CO2(g),;ΔHrx=42.5 Kcal/gm mole (891,764 Kcal/mt CaO)
The rotary kiln calcines the mud between 1000° C. (1832° F.) and 1200° C. (2192° F.) and at CO2 partial pressures well below the atmospheric pressure equilibrium concentration for these temperatures. This produces a calcined lime mud having the best physiochemical properties suitable for subsequent slaking and efficient recausticizing.
Due to the high calcination temperatures, and so to not contaminate and/or upset the recausticizing process with inorganic impurities, either high-cost oil and/or natural gas fuels are utilized as kiln fuel. Low-cost solid fuels such as biomass, waste water treatment plant (WWTP) sludge, coal etc. are typically not used due to their contaminating ash content. WWTP sludge and biomass have the added penalty of lower adiabatic flame temperature due to the high water content.
Accordingly, while many energy-intensive pulp mill operations have converted to low-cost waste and biomass fuels for energy production since the 1970s, the rotary kiln remains a conspicuous consumer of premium liquid and gaseous fuels. While advances have been made to reduce this premium fuel consumption, it still remains between 1.4 (with LMD) and 1.7 million Kcal /metric ton calcined lime mud dependent on initial mud moisture content, calciner capacity, fuel type, product lime availability, and installed energy conservation features.
Due to various limitations, attaining future significant fossil fuel consumption and cost reductions in the rotary kiln/LMD calcination process appears difficult. There is, however, wasted energy within the rotary kiln/LMD calcination process that could be recovered with the proper technical approach. Notably, at higher lime mud solids concentration the calciner's exit gas temperature increases. If a counter-current heat transfer process (i.e. a rotary kiln) were thermally balanced the exit gas temperature would remain constant as fuel input was reduced to compensate for the decreased water input.
Further energy efficiency improvement, however, is not likely with the rotary kiln/LMD calcination process since a very large non-variable fuel amount is required to compensate for constant radiation losses, provide the constant endothermic heat-of-reaction enthalpy, and also heat reaction products (CaO and CO2) to the calcination temperature. This non-variable fuel input has associated gaseous fuel combustion products from which enthalpy is recovered via counter-current contact with dry lime mud solids in the kiln pre-heat section using densely packed hanging chains as heat transfer surface. This preheats the dry lime mud before it enters the following kiln calcination stage.
The reduced temperature gaseous combustion products (and released CO2) leave the kiln pre-heat section and enter the kiln drying section where these gases' enthalpy content evaporates incoming lime mud water content. Older kilns have chains within the kiln drying section to improve gas-to-water heat transfer. Newer kilns with an LMD do not have drying section chains and are easier to control and operate. As previously stated, as lime mud solids content increases the need for drying enthalpy decreases. The following kiln pre-heat section, however, has insufficient chain heat transfer ability to absorb available enthalpy from the combustion products and CO2 associated with the aforementioned non-variable fuel component and transfer it into the dried solids entering from the drying zone. The unabsorbed enthalpy associated with the combustion products and CO2 results in a higher LMD outlet gas temperature when high solids lime mud is the feedstock. Over the last forty years, improvements in lime mud filtration and washing have increased filter cake solids content from 70% to over 85%, resulting in significant fuel savings and improved white liquor recovery. Unfortunately, the current rotary kiln/LMD technology is limited in the ability to economically respond to this fuel saving opportunity and will become less fuel-efficient as filter cake solids content further increases.
The less utilized FluoSolids fluidized bed calcination process never featured a solids pre-heat section, and wastefully dissipated this excess available enthalpy via a water spray cooler to control lime mud flash dryer operating temperature. Designs have also been proposed to address this dilemma by inserting a waste heat boiler in place of the spray cooler step, but were never commercialized, most likely due to the high surface fouling characteristics of calciner exit gas caused by the presence of low eutectic melting point mixtures of Na2CO3 and sodium Na2SO4. The Na2SO4 is created by the oxidation of Na2S2O3 and Na2S in the air fluidized calciner.
It would, therefore, be beneficial to provide a process whereby gaseous fuel combustion products could be separated from gaseous calcination reaction products (CO2) such that the excess enthalpy contained in the combustion products could be viably extracted as superheated high pressure steam without the presence of heat transfer fouling mixtures such as Na2CO3/Na2SO4. This is not possible within the body of a rotary kiln however the disclosed invention, with separated combustion and calcination stages, addresses this need.
Concurrent with these enthalpy utilization optimization needs, all mills must control the amount and toxicity of gaseous, liquid, and solid wastes expelled. Many of these emissions have been reduced or eliminated thanks to better manufacturing practices but WWTP sludge (cellulosic, organic, and inorganic matter from waste water treatment) remains a costly disposal problem since it must ultimately be placed in a landfill. As previously discussed, WWTP sludge cannot be used in existing rotary kiln representing a lost opportunity to conserve fossil fuels.
Safe disposal of non-condensable waste mill gas (NCGs), which are typically combusted in the recovery or power boiler, or more likely, the rotary kiln lime mud calciner is another Kraft paper pulp mill operability issue. While NCG combustion in rotary kilns has been widely practiced, operability problems (kiln deposit “ringing”, SO2 “blow-through”, etc.) persist at many mills. Accordingly, stand alone NCG incinerators with waste heat boilers that raise steam and scrub sulfurous emissions are increasingly used. These incinerator/boilers, however, are not always available when NCGs are produced, so a back-up disposal means is desirable.
Numerous advances have been previously made related to various aspects of lime mud and limestone calcination and related materials. U.S. Pat. No. 2, 212,446 teaches limestone calcination in a 100% steam atmosphere (a claim of the disclosed invention) using an indirect heated rotary calciner. U.S. Pat. No. 2,700,592 teaches using moving media heat transfer (MMHT) between an endothermic fluidized bed process and an exothermic fluidized bed sulfide ore roasting process. U.S. Pat. No. 2,738,182 teaches fluidized bed calcination of Kraft pulp mill lime mud including recycling finely ground calcined lime mud product into a calciner bed to control agglomeration. U.S. Pat. No. 3,961,903 teaches a spray dryer to dry lime mud using multiple hearth calciner off-gases as the drying medium prior to feeding the dried mud to the calciner. U.S. Pat. No. 3,991,172 teaches direct combustion products calcination of fine limestone by passing the limestone through a fluidized bed of a “granular heat carrier medium”. U.S. Pat. No. 4,321,239 teaches using multiple spray dryers to dry lime mud using multiple hearth calciner off-gases as the drying medium prior to feeding the dried lime mud to a calciner. U.S. Pat. No. 4,389,381 teaches using MMHT by passing fine limestone through an inert heat carrier contained in an endothermic fluidized bed and using a coal fueled exothermic fluidized bed to re-heat the heat carrier. Ash is separated from the re-heated heat carrier prior to calcination. Calcination is accomplished in an air atmosphere of unspecified composition. U.S. Pat. No. 4,606,722 teaches a solid fuel gasified external to a rotary kiln lime mud calciner with the syngas used as calciner fuel. A vitrified gasifier ash is mixed with calcine and removed in the slaker. U.S. Pat. No. 4,631,025 teaches direct injection of a solid fuel (petroleum coke) into a fluidized bed lime mud calciner. U.S. Pat. No. 4,707,350 teaches calcination of fine limestone in an electrically heated fluid bed calciner fluidized in a 100% CO2 atmosphere with recovered CO2 as the fluidizing gas. U.S. Pat. No. 4,760,650 teaches indirect steam heated drying of lime mud in a steam atmosphere prior to feeding the dried lime mud into a fluid bed calciner. The steam is generated from calciner off-gas. U.S. Pat. No. 5,110,289 uses a separate flash dryer to dry Kraft paper pulp mill lime mud using rotary calciner off-gases as the drying medium. U.S. Pat. No. 5,230,880 teaches calcination of fine limestone in an electrically heated fluid bed calciner fluidized in an air atmosphere. The fine limestone is passed through a bed of coarser calcined limestone particles that act as a heat transfer media between the fine limestone and the electric heaters. U.S. Pat. No. 5,354,375 describes a lime mud calcination process using a shaft kiln to process pelletized lime mud in a counter-current fashion using direct firing of oil or natural gas fuel. U.S. Pat. No. 5,378,319 describes a lime mud calcination process using an electrically heated microwave belt oven to process lime mud in a counter-current fashion using a counter-current air sweep. U.S. Pat. No. 5,644,996 teaches a technique to cool freeboard gases in a fluidized bed lime mud calciner to below 500° C. (932° F.) to minimize freeboard scaling when the calciner fluid bed is between 875° C. (1607° F.) and 1000° C. (1832° F.). The injected coolant is the entire amount of wet lime mud. U.S. Pat. No. 5,653,948 teaches an indirectly heated fluid bed calciner using electricity or oil/gas firing to calcine very fine limestone particles. The limestone is injected beneath a coarser limestone bed that acts as the heat transfer medium. U.S. Pat. No. 5,711,802, teaches a technique to reduce the LMD inlet gas temperature from a rotary kiln lime mud calciner to between 400° C. (752° F.) and 600° C. (1112° F.); that eliminates dryer scaling and reduces kiln dust carry-over. United States Patent Application Publication No. 2006/0039853 teaches a process to separate CO2 from utility boiler stack gases with an “activated” CaO sorbent and then separately re-generating the sorbent and recovering the CO2 in a steam blanketed vacuum calciner.